This invention relates to apparatus for producing X-rays and more particularly to X-ray sources of the form having electron beam producing means, a plurality of different target-anodes and deflection means for directing the electron beam towards any selected one of the targets to produce X-rays having selectable characteristics.
Prior copending U.S. application Ser. No. 353,451 of the present Applicant, filed Apr. 24, 1973 and entitled "SELECTABLE WAVE LENGTH X-RAY SOURCE, SPECTROMETER AND ASSAY METHOD", which issued Dec. 9, 1975 as U.S. Pat. No. 3,925,660, discloses a highly advantageous form of X-ray source in which an X-ray beam may be selectively directed to any of a number of spaced apart target anodes in order to vary the wavelength spectrum or some other characteristic of the X-ray output. The general usefulness of an X-ray source is greatly enhanced if the output may be selectively varied. In medical radiology, for example, the particular wavelengths which are optimum for producing a photographic or fluoroscopic image in which bone structure is to be distinguished from the soft tissue of a patient may be different from the wavelengths which are most suitable for distinguishing tumors from healthy tissue. In checking the interior structure of castings or other manufactured parts for structural flaws by X-ray analysis, different wavelengths may be preferable for checking parts made of different materials or which have different shapes. An ability to change wavelengths is equally helpful in many other usages of X-ray sources.
In an X-ray source of this kind, the electron beam is switched from one target to another by applying appropriate deflection signals to a magnetic or electrostatic beam deflection means or in some cases to both. Once a particular target has been selected, it is desirable that the path of the electron beam remain fixed until such time as it is desired to change the characteristics of the X-ray output. In some instances it is not only desirable that the electron beam be maintained within the area of the selected target, but also that the particular point of impact of the beam on that target remain constant. In radiography, for example, the resolution obtainable in a photographic or fluoroscopic image is dependent in part on constancy of the point of origin of X-rays at the selected target. In X-ray spectrometry for determining the elemental composition of materials or in X-ray monitoring of materials for thickness variations, constancy of the X-ray origin point may be important in order to maintain a constant distribution of radiation across the material and in order to provide for reproducible results. In this way any non-uniformities in the region of the material being irradiated are always averaged out in the same way.
The energy of the electron beam within the source may vary either through random voltage fluctuations in the beam acceleration system or because it may be deliberately changed for any of various purposes. It may be desirable to adjust beam energy upon switching to a newly selected target, for example, to minimize bremmstrahlung radiation and thereby produce a substantially monochromatic X-ray output. Such changes of electron beam energy can complicate operation of the X-ray source in that a change of beam energy causes a shift in the point of impact of the electron beam at the selected target unless a compensating adjustment is also made in the deflection signal.
The necessary adjustment of the deflection signal may not always be proportional to the change of beam energy but may instead be a non-linear function of energy. Beam energy is strictly proportional to the accelerating voltage difference which is applied between the cathode and the target anodes of the source but the deflection signal magnitude required to direct the beam to a given point is not. Deflection signal magnitude is dependent on electron beam velocity which is not itself a linear function of beam energy or accelerating voltage.
Thus, it has frequently been necessary to make complicated deflection signal adjustments when beam energy is changed for operational purposes, such as in switching from one target to another. Insofar as indeliberate fluctuations in beam accelerating voltage are concerned, slow drift can be corrected for by monitoring a voltage reading instrument in order to make manual adjustments as necessary, but it is not practical to attempt to compensate for brief monentary perturbations of the accelerating voltage and beam trajectory in this manner. The general utility of multitarget X-ray tubes and ease of operation of such devices can be greatly enhanced if beam energy adjustments can be made or allowed to occur without requiring compensating manual adjustments in other controls.